A pixel sensor comprises an array of pixel sensor cells that detects two dimensional signals. Pixel sensors include image sensors, which may convert a visual image to digital data that may be represented by a picture, i.e., an image frame. The pixel sensor cells are unit devices for the conversion of the two dimensional signals, which may be a visual image, into the digital data. A common type of pixel sensors includes image sensors employed in digital cameras and optical imaging devices. Such image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While complementary metal oxide semiconductor (CMOS) image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
For typical CMOS image sensors, the images are captured employing a “rolling shutter method.” FIG. 1 shows a typical prior art rolling shutter image capture and read out sequence. In the rolling shutter method, the imaged is captured on a row-by-row basis within a pixel array, i.e., the image is captured contemporaneously for all pixels in a row, but the capture of the image is not contemporaneous between adjacent rows. Thus, the precise time of the image capture is the same only within a row, and is different from row to row.
For each pixel in a row, the image is captured in its light conversion unit, which is a photosensitive diode. Charges generated from the light conversion unit are then transferred to a floating diffusion node. The amount of charge stored in the floating diffusion node is then read out of each pixel via a transistor wired in the source follower configuration whose gate is electrically connected the floating diffusion node. The voltage on the source of said source follower is then read out to column sample circuits, thereby completing the read out of all the pixels in the selected row, before moving on to the next row. This process is repeated until the image is captured by the pixels in all the rows, i.e., by the entire array of the pixels.
Since the same column sample circuits are employed to read out the data row by row without delay between the exposure and the read out, the read out of the rows of the image sensor is staggered between rows. Therefore, the exposure of the image sensor needs to be staggered row by row. In other words, different rows are exposed at different times. The resulting image is one where the each row captured actually represents the subject at a different time. Thus, for highly dynamic subjects (such as objects moving at a high rate of speed), the rolling shutter methodology can create image artifacts.
To solve this image artifact issue of capturing high speed objects, a global shutter method may be employed. FIG. 2 shows a typical prior art global shutter image capture and read out sequence. The global shutter method employs a global shutter operation, in which the entirety of the array of image sensors is reset prior to exposure simultaneously. The image for the whole frame is captured in the light conversion units of the pixels at the exactly same time for all the rows and columns. The signal in each light conversion unit is then transferred to a corresponding floating diffusion node. The voltage at the floating diffusion nodes is read out of the imager array on a row-by-row basis. The global shutter method enables image capture of high speed subjects without image artifacts, but introduces a concern with the global shutter efficiency of the pixel since the integrity of the signal may be compromised by any charge leakage from the floating diffusion node between the time of the image capture and the time of the reading of the imager array.
Specifically, in the rolling shutter method, the image signal is held at the floating diffusion node (FD) for a significantly shorter time than the actual time of exposure in the light conversion unit, e.g., a photodiode. Thus the contribution of the generation rate of the FD is orders of magnitude smaller than the generation rate during the integration time in the light conversion structure, e.g., the photodiode.
In contrast, the image signal is held at the FD for varying amounts of time in the global shutter method. For example, the signal from the first row may have the least wait time, which is the time needed to read out a single row. The signal from the last row has the greatest wait time which corresponds to the full frame read-out time, which is equal to the product of the number of rows in the array with the time needed to read out a single row. The charge on the floating diffusion may be degraded due to charge leakage or charge generation during the wait time for the last row. Any charge generations or charge leakage that occurs on the floating diffusion node during the wait time can have a significant impact to the quality of the signal that is read out of the imager.
Referring to FIG. 3, a prior art CMOS pixel sensor cell comprises a semiconductor substrate 8 and a transfer gate transistor formed thereupon. The semiconductor substrate 8 comprises a heavily-doped first conductivity type semiconductor layer 10, a lightly-doped first conductivity type semiconductor layer 12, an isolation structure 20 which may be shallow trench isolation, LOCOS, or other semiconductor isolation, and a surface pinning layer 34.
The heavily-doped semiconductor layer 10 comprises a heavily doped semiconductor material having a first conductivity type doping. The first conductivity type is p-type or n-type. The lightly-doped first conductivity type semiconductor layer 12 comprises a lightly-doped semiconductor material having the first conductivity type doping, which is a low concentration doping with first conductivity type dopants. The surface pinning layer 34 has a doping of the first conductivity type.
The semiconductor substrate 8 further comprises a second conductivity type charge collection well 30. A lightly-doped first conductivity type region 32 is a portion of the lightly-doped first conductivity type semiconductor layer 12 located directly underneath the second conductivity type charge collection well 30. The lightly-doped first conductivity type region 32 typically has the same dopant concentration as the rest of the lightly-doped first conductivity type semiconductor layer 12.
The lightly-doped first conductivity type region 32 and the second conductivity type charge collection well 30 collectively constitute a photodiode (32, 30) that generates electron-hole pairs. Charge carriers of the second conductivity type are collected in the second conductivity type charge collection well 30 in proportion to the amount of photons impinging into the photodiode (32, 30). Electron-hole pairs are generated within the depletion region of the photodiode (32, 30), due to photogeneration processes. Particularly, if the carrier is a carrier of the second conductivity type, the carrier accumulates in the second conductivity type charge collection well 30. The amount of charge that accumulates in the second conductivity type charge collection well 30 is nearly linear to the number of incident photons (assuming the photons have the same energy distribution).
The transfer gate transistor comprises a gate dielectric 50, a gate electrode 52, a gate spacer 58, a source, which is the second conductivity type charge collection well 30, and a drain, which is herein referred to as a floating drain 40. Specifically, the transfer gate transistor is integrally formed with the photodiode (30, 32) such that the second conductivity type charge collection well 30, which comprises a lightly-doped second conductivity type semiconductor material, is also a source of the transfer gate transistor. The second conductivity type is the opposite of the first conductivity type. For example, if the first conductivity type is p-type, the second conductivity type is n-type, and vice versa.
The floating drain 40 has a doping of the second conductivity type, and is electrically floating when the transfer transistor is turned off to enable storage of electrical charges. A first conductivity type well region 42 is formed by implantation of first conductivity type dopants under the floating drain 40.
Charge carriers of the second conductivity type, which are electrons or holes, accumulate in the second conductivity type charge collection well 30 when photons are incident on the photodiode (32,30). When the transfer transistor is turned on, the electrons in the second conductivity type charge collection well 30 are transferred into the floating drain 40, which is a charge collection well and stores electrical charge from the photodiode (30, 32) as data until a read circuit detects the amount of stored charge. Thus, the second conductivity type charge collection well 30 functions as the source of the transfer transistor while the transfer transistor is turned on. The turn-on of the transfer transistor corresponds to the transfer of the entire array from photosensitive diode to floating diffusion as described in FIG. 2.
In general, a difficulty in global shutter imaging is that the charge needs to stored in the floating diffusion 40 for a long time—up to the read out time for the entire frame which can be up to a tenth of a second or more. During this time, the leakage on the diffusion directly impacts the image quality. Obtaining high quality digital images in the global shutter operation scheme requires preservation of the charge in the floating drain 40 without any significant change in the amount of stored charge until the read out. The greater the leakage current of the floating drain, the greater the change in the amount of charge between the transfer from the second conductivity type charge collection well 30, which is a terminal of the photosensitive diode (30, 32), and the read out. Most leakages are time dependent and are characterized by a rate measured in an amount of charge leaked to or from the diffusion per unit time.
Since images are typically read out from top to bottom, the data from the top row of the image will be on the diffusion for a very short time before being read out and therefore very little noise will be added to this row due to leakage on the read out diffusion. This will gradually get worse to the bottom of the image. The data on the bottom row of the image will sit on the diffusion for the full read time of the frame and thus will have the largest leakage current. Thus, rolling shutter images are of worse quality at the bottom than the top. Leakage both creates a loss of contrast as well as fixed pattern noise, and both of these can be visibly worse at the bottom of the image. The human eye is very sensitive to such correlated noise and images which appear worse at one side are unacceptable for consumer photography.
Further, the amount of data distortion and the loss of image fidelity are also affected by local variations in the leakage current and the voltage at the floating drain of a CMOS pixel sensor cell, which depends on the amount of charge stored therein.
In view of the above, there exists a need to provide a method of alleviating the impact of image degradation due to the variations in the charge hold time among the different rows of an array of a CMOS image sensor operated in global shutter mode.
Further, there exists a need to provide a method for compensating for the leakage current to improve the signal-to-noise ratio of the image frame of the array of CMOS image sensors operated in global shutter mode.